1. Field of the Invention
The present invention relates to a liquid crystal display device, a backlight used for a same display device, a method for driving a same backlight and a method for manufacturing a same backlight.
The present application claims priorities of Japanese Patent Application No. 2001-324873 filed on Oct. 23, 2001, which are hereby incorporated by reference.
2. Description of the Related Art
In recent years, a remarkable improvement in performance of a liquid crystal display device (hereinafter being referred to an “LCD”) is yielded and also a remarkable progress in making its screen larger is made. The LCD having a large screen is used as a monitor for a personal computer or a like. Development work of a liquid crystal television by expanding technology of the LCD with a large screen is actively proceeding.
In such an application to the television, an improvement in performance of displaying a moving picture in an LCD is strongly required. There are two main reasons for dissatisfactory performance of a conventional LCD.
The first reason is that a response speed in the LCD is low. The second reason is that display using the LCD is performed by using illuminating light being applied constantly. Hereinafter, the second reason is described in detail. In ordinary cases, in a CRT (Cathode Ray Tube) which performs display of moving pictures, a movie or a like, a period of non-display is provided between displaying time for a screen and for a subsequent screen.
In the CRT, an image screen is produced by scanning using an electron beam on a fluorescent material. As a result, fluorescent light for a pixel disappears after being scanned and does not appear until a subsequent screen scanning period starts.
Moreover, in the case of a movie, due to a period required for feeding a film existing between displaying time for a screen and a subsequent screen, illuminating light is intercepted, in ordinary cases, during this period.
On the other hand, in the case of the LCD, since light fed from a backlight is applied constantly, a non-display period between displaying time for a screen and for a subsequent screen does not exist. Therefore, even if a moving picture is displayed using the LCD, the moving pictures look like as if they shake.
To solve this problem, a method is proposed in which light to be fed from the backlight is applied in synchronization with timing of scanning on the liquid crystal display screen. This method is disclosed in a-literature, for example, “First Response Liquid Crystal Display” (by Taira et al., AM-LCD 1998, p113-p 116). In this case, the backlight is made up of an LED (Light Emitting Diode) and is lit in synchronization with timing of scanning on the LCD. This causes a scanning screen like a CRT to be produced in a pseudo manner, thereby trying to improve performance of displaying moving pictures.
For example, a backlight assembly to be used for an LCD having a fluorescent layer disclosed in Patent Application Laid-open No. Hei 9-258227 is provided with a plurality of cold cathode fluorescent tubes having a length being equivalent to a liquid crystal panel and being stacked in layers in parallel and a pair of supporting plates adapted to support a fluorescent tube being coupled to an end of a cold cathode fluorescent tube, in which the fluorescent tube is sequentially lit to form a consecutive image on a screen and, in order to excite a fluorescent material (phosphor) contained in a fluorescent layer, light having a magenta color with a wavelength of 380 nm to 420 nm is emitted.
Moreover, Japanese Patent Application Laid-open No. Hei 10-10997 discloses a method for driving a display device which places a plurality of line-shaped light sources for emitting each of R, G, and B colors on a transparent light guiding plate made of an acrylic resin so that light having each of the colors extends in a scanning line direction and having a backlight device with a width of the line-shaped light sources for emitting each of the R, G, and B colors being equivalent to several tens of horizontal pixel lines employed in the liquid crystal panel in which, when the horizontal pixel line of the display panel is selected for scanning, the line-shaped light source corresponding to the driving line emits light having each of colors corresponding to its color signal.
These conventional backlight units adapted to apply light fed from the backlight in synchronization with timing of scanning on the LCD present problems in that component counts increase and a rise in costs for fabrication are unavoidable. That is, when the LED is used, since many LEDs have to be arranged on a surface of the backlight, the increase in component counts and in assembling processes are unavoidable. This presents a serious problem in the case of a large-screen-type LCD in particular.
Moreover, even when a plurality of cold cathode fluorescent tubes is used, as a size of a screen increases, a number of the cold cathode fluorescent tubes increases. Therefore, a price of the backlight rises in the case of the large-screen-type LCD.
Thus, in the conventional scanning-type backlight, though performance of displaying a moving picture is improved, costs for fabricating the backlight are high and therefore it is impossible to make low a price of the LCD device.